Beat gettin' shit on, toilet bowl seat

from Mac Miller – Cold Feet Lyrics on Rap Genius

To shit on means to kill or destroy a beat with his raps like Mac does here. This line is also rapped in a style that many others, most notably Weezy and Rick Ross use where they say a phrase then end the line with something related to what they said first

http://rapgenius.com/Lil-wayne-6-foot-7-foot-lyrics

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